Properties of a deep seismic waveguide measured with an optical fiber

Low-velocity zones located deep in the subsurface can act as seismic waveguides. Traditionally, their experimental observation has been limited by the practical challenges of in situ recording. We use a measurement technique in which optical fibers are turned into seismic sensors. The fiber is deployed along a horizontal well drilled inside a 15-m-thin shale formation at a depth of about 2 km. Owing to the high-resolution recording of the optical fiber, we can distinctly observe three previously elusive guided wave modes over a wide frequency range. As their propagation is primarily confined to the waveguide and strongly depends on its seismic properties, such guided waves hold tremendous potential for high-resolution imaging of deep low-velocity structures, such as fault zones, saline aquifers, and hydrocarbon reservoirs.

Figure 1

(a) Experimental setup showing the fiber-instrumented well (black) and the offset well (red). Both wells are perforated. (b) Logging results of a vertical well (schematically marked in orange) including P (red) and S (blue) velocities, density (pink), and inverse of the shale content estimated by gamma rays. The 15-m-thick reservoir boundaries are denoted by dashed black lines. (c) Seismic velocity logging conducted in the horizontal section of the DAS-instrumented (black) well. P-wave (red), SH-wave (cyan), and SV-wave (blue) velocities are shown. Shear-wave splitting due to anisotropy is evident, and the P-wave velocity is much higher than that measured by the vertical log in the reservoir interval.

Figure 2

Snapshots of the horizontal component of wave propagation in a seismic waveguide. Images are taken 0.1 s apart. The seismic velocities of the three layers are marked in the top snapshot. Blue/orange: Propagation above the waveguide. Black/yellow: In the waveguide. Pink/green: Below the waveguide. Two distinct types of waves confined to the low-velocity reservoir (marked in dotted black lines) are visible: Leaky (red arrow) and P-SV normal (purple arrow) modes. For the leaky modes, there is a strong S-wave radiation into the layers above and below the reservoir (dotted red arrows). The normal modes decay exponentially away from the boundaries outside the reservoir, with a wavelength-dependent skin depth.

Figure 3

Guided-wave dispersion curves in a three-layer model. (a) Results of 2D isotropic wave-equation modeling. P-SV modes (pointed by green arrow) and leaky modes (pointed by red arrow) are visible. For both, there are multiple propagation modes. They are marked as 0 (fundamental), 1, and 2 for normal P-SV waves. (b) Analytical solution for the 3D isotropic case. The leaky P waves (red) have a discontinuous spectrum. Both leaky and normal P-SV modes (green) match the wave-equation solution shown in (a). The normal SH mode (blue) differs from the P-SV case. (c) Analytical solution for the 3D case with an anisotropic low-velocity layer. Normal P-SV modes are almost unchanged and are still bounded by the S-wave velocity in the vertical direction. SH modes are bounded by the horizontal velocity of the layer, which is faster. Leaky modes are bounded by the P-wave velocity in the horizontal direction, which is faster than the vertical; as a result, only three continuous modes are present.

Figure 4

Field data observations. We show time-domain records of a single perforation shot along the DAS-instrumented well (a) and in the offset well (b), after mapping to horizontal source-channel distance. Leaky (red), normal SH (cyan), and normal P-SV (green) modes can be observed. Dispersion images obtained by summation of 20 sources along the DAS-instrumented well (c) and 43 sources in the offset well (d) show the dispersive properties of the different modes. There are two main leaky branches (red) in which the spectrum is discontinuous, two normal SH branches (blue), and a single normal P-SV branch.